1. Field of the Invention
The present invention relates to a phosphorescent multinuclear copper complex and an organic electroluminescent (EL) device using the same, and more particularly, to a phosphorescent multinuclear copper complex emitting light in the yellow to red wavelength region of 560 nm to 630 nm and an organic electroluminescent device including the multinuclear copper complex as an organic layer forming material.
2. Description of the Related Art
Organic electroluminescent (EL) devices are active display devices using the phenomenon of light generation occurring due to the recombination of electrons and holes in a fluorescent or phosphorescent organic compound thin layer (hereinafter, organic layer) when a current is applied to the organic layer. The organic electroluminescent (EL) devices are lightweight, include simpler and less parts, have a structure that can be manufactured through simple processes, produce high-quality images, and have a wide viewing angle. Organic electroluminescent devices also can produce high-color purity moving pictures, and have low power consumption and a low driving voltage. Therefore, organic electroluminescent devices have electrical characteristics suitable for portable electronic devices.
In general, an organic electroluminescent device has a structure including an anode, a hole transporting layer, an emitting layer, an electron transporting layer, and a cathode, which are sequentially stacked on a substrate. The hole transporting layer, the emitting layer, and the electron transporting layer are organic layers formed of organic compounds. The operating principle of the organic electroluminescent device having such a structure as described above is as follows. When a voltage is applied between the anode and the cathode, holes injected from the anode move to the emitting layer via the hole transporting layer. Electrons are injected from the cathode to the emitting layer via the electron transporting layer. Excitons are generated due to the recombination of carriers in the emitting layer. The excitons undergo radiative decay, emitting light having a wavelength corresponding to the band gap of a material.
Materials for forming the emitting layer of the organic electroluminescent device are classified into fluorescent materials using singlet-state excitons and phosphorescent materials using triplet-state excitons according to the emission mechanism. The emitting layer is formed by doping a fluorescent material or a phosphorescent material directly or doping a fluorescent material or a phosphorescent material on an appropriate host material. As a result of the electron excitation, singlet excitons and triplet excitons are generated in the host. Here, a statistical generation ratio between the singlet excitons and triplet excitons is 1:3 (Baldo, et al., Phys. Rev. B, 1999, 60, 14422).
In an organic electroluminescent device using a fluorescent material as a material for forming the emitting layer, triplet excitons that are generated in the host cannot be used. However, in an organic electroluminescent device using a phosphorescent material as a material for forming the emitting layer, both singlet excitons and triplet excitons can be used, and thus, a 100% internal quantum efficiency can be obtained (Baldo et al., Nature, Vol. 395, 151-154, 1998). Accordingly, the use of a phosphorescent materials leads to a higher light emitting efficiency than when a fluorescent material is used.
When a heavy metal, such as Ir, Pt, Rh, or Pd is included in an organic molecule, spin-orbita coupling occurs due to a heavy atom effect, and thus, singlet excitons and triplet excitons are mixed, thereby enabling the transition and thus effective phosphorescence even at room temperature.
Various materials using a transition metal compound containing a transition metal, such as Iridium (Ir), platinum (Pt), etc. have been reported as high-efficient luminescent materials exhibiting phosphorescence. However, a phosphorescent material emitting light in the yellow to red wavelength region of 560 nm to 630 nm is still required for a high-efficiency, full-color display device.
Pyrazolate ligands are important in the coin metal chemistry. Pyrazolate ligands form a multinuclear complex by coordinating to a metal ion, such as Cu (I), Ag(I), Au(I), etc., in exo-bidentate mode. Coin metal pyrazolates can form a trimer, a tetramer, a hexamer, and up to a polymer according to the reaction condition and the substituent in the pyrazolate moiety. Pyrazolate ligands improve the performance of an organic EL device by functioning as electron transporting moieties assisting the injection of electrons.
Among such coin metal pyrazolates, a multinuclear coin metal having a fluorinated pyrazolate ligand has very interesting light emitting characteristics. Fluorination facilitates thin film formation by assisting volatilization, improves thermal stability and stability of oxidation, and leads to a decrease in emission concentration quenching.
Mohammad (Mohammad A. Omary, Inorg. Chem., 2003, 42, 8612) discloses a metal pyrazolate complex with 2,4,6-cholidine coordinated to copper atoms. This complex emits bright blue light.
In addition, there is a continuous need for the fluorinated metal pyrazolate complex compounds that contain various ligands coordinated to the metal atom in the fluorinated metal pyrazolate complex and that have excellent light emitting characteristics at non-blue wavelength regions.